1. Field of the Invention
This invention relates to a scanner, and more particularly, to a method for adjusting scanning speed of a scanner.
2. Description of the Prior Art
Scanners are commonly used in office environment for scanning documents into computers. A scanner usually comprises a scanning module movably mounted over a shaft for scanning a document placed above the scanner, a step motor for moving the scanning module step by step, a memory buffer for storing image data scanned from the document, and a control unit for controlling the operations of the scanner and also communications with a host computer connected to the scanner. Within each step of the step motor, one line of the document is scanned by the scanning module and the image data is stored in the memory buffer. And then the control unit transmits the image data stored in the memory buffer to the host computer through an I/O cable for further processing.
The data transmission rate from the scanner to the host computer is depended on the communication protocol used between the scanner and the host computer, maximum scanning speed of the scanner, and the processing speed of the host computer. If the transmission rate is higher than the scanning speed of the scanner, the scanner can run in full speed until a document is completely scanned. But if the transmission rate is slower, image data will be accumulated in the memory buffer and it may get full after portion of a document is scanned. When such condition happens, the scanning module must be stopped to avoid possible loss of the image data. After the image data accumulated in the memory buffer is later on transmitted to the host computer, the scanning module will then be resumed by the control unit to scan the document from where it was stopped.
One problem usually occurs when the step motor is resumed to move the scanning module. For the first few steps the motion of the scanning module is usually quite shaky and unstable which may affect the quality of the scanning module's output during this period. In order to solve such problem, two methods have been developed in the past. The first method is to move the scanning module backward for a predetermined number of steps and then forward the step motor from there. The image data generated over the first few steps will be ignored and when the scanning motor reaches the last stopped position, the motion of the scanning module has become stabilized and the image data will be picked up and stored in the memory buffer. This method solves the unstable problem of the scanning module but it takes time to move the scanning module backward and forward which may delay the overall scanning speed of the scanner. Besides, certain mismatch over the last stopped position generated by such method may also affect the quality of the image data generated over the last stopped position.
The second method is to increase the size of the memory buffer so that more image data can be stored in the memory buffer and the scanning module can scan a document continuously without any temporary pause. Enlarging memory buffer to solve this problem is quite straight forward and easy but it has two drawbacks: it increases both the cost and complexity of a scanner. DRAM (dynamic random access memory) chips are usually used to increase memory buffer size for its low cost. By using DRAM, extra memory refreshing circuit will have to be implemented which makes the memory circuits of the scanner more complex. Besides, increasing memory buffer may not really solve the problem. Current scanners usually allow high resolution color image scanning. Such scan can generate a huge amount of image data within a very short period of time, and it is very unreasonable to simply increase the memory buffer size to alleviate this problem because the increased cost of the scanner can not justify the benefit of such design.